The term speech coding refers to the process of compressing and decompressing human speech. Likewise, a speech coder is an apparatus for compressing (also referred to herein as coding) and decompressing (also referred to herein as decoding) human speech. Storage and transmission of human speech by digital techniques has become widespread. Generally, digital storage and transmission of speech signals is accomplished by generating a digital representation of the speech signal and then storing the representation in memory, or transmitting the representation to a receiving device for synthesis of the original speech.
Digital compression techniques are commonly employed to yield compact digital representations of the original signals. Information represented in compressed digital form is more efficiently transmitted and stored and is easier to process. Consequently, modern communication technologies such as mobile satellite telephony, digital cellular telephony, land-mobile telephony, Internet telephony, speech mailboxes, and landline telephony make extensive use of digital speech compression techniques to transmit speech information under circumstances of limited bandwidth.
A variety of speech coding techniques exist for compressing and decompressing speech signals for efficient digital storage and transmission. It is the aim of each of these techniques to provide maximum economy in storage and transmission while preserving as much of the perceptual quality of the speech as is desirable for a given application.
Compression is typically accomplished by extracting parameters of successive sample sets, also referred to herein as “frames”, of the original speech waveform and representing the extracted parameters as a digital signal. The digital signal may then be transmitted, stored or otherwise provided to a device capable of utilizing it. Decompression is typically accomplished by decoding the transmitted or stored digital signal. In decoding the signal, the encoded versions of extracted parameters for each frame are utilized to reconstruct an approximation of the original speech waveform that preserves as much of the perceptual quality of the original speech as possible.
Coders which perform compression and decompression functions by extracting parameters of the original speech are generally referred to as parametric coders or vocoders. Instead of transmitting efficiently encoded samples of the original speech waveform itself, parametric coders map speech signals onto a mathematical model of the human vocal tract. The excitation of the vocal tract may be modeled as either a periodic pulse train (for voiced speech), or a white random number sequence (for unvoiced speech). The term “voiced” speech refers to speech sounds generally produced by vibration or oscillation of the human vocal cords. The term “unvoiced” speech refers to speech sounds generated by forming a constriction at some point in the vocal tract, typically near the end of the vocal tract at the mouth, and forcing air through the constriction at a sufficient velocity to produce turbulence. Speech coders which employ parametric algorithms to map and model.
There are several types of vocoders on the market and in common usage, each having its own set of algorithms associated with the vocoder standard. Three of these vocoder standards are:                1. LPC-10 (Linear Prediction Coding): a Federal Standard, having a transmission rate of 2400 bits/sec. LPC-10 is described, e.g., in T. Tremain, “The Government Standard Linear Prediction Coding Algorithm: LPC-10,” Speech Technology Magazine, pp. 40–49, April 1982).        2. MELP (Mixed Excitation Linear Prediction): another Federal Standard, also having a transmission rate of 2400 bits/sec. A description of MELP can be found in A. McCree, K. Truong, E. George, T. Barnwell, and V. Viswanathan, “A 2.4 kb/sec MELP Coder Candidate for the new U.S. Federal Standard,” Proc. IEEE Conference on Acoustics, Speech and Signal Processing, pp. 200–203, 1996.        3. TDVC (Time Domain Voicing Cutoff): A high quality, ultra low rate speech coding algorithm developed by General Electric and Lockheed Martin having a transmission rate of 1750 bits/sec. TDVC is described in the following U.S. Pat. Nos. 6,138,092; 6,119,082; 6,098,036; 6,094,629; 6,081,777; 6,081,776; 6,078,880; 6,073,093; 6,067,511. TDVC is also described in R. Zinser, M. Grabb, S. Koch and G. Brooksby, “Time Domain Voicing Cutoff (TDVC): A High Quality, Low Complexity 1.3–2.0 kb/sec Vocoder,” Proc. IEEE Workshop on Speech Coding for Telecommunications, pp. 25–26, 1997.        
When different units of a communication system use different vocoder algorithms, transcoders are needed (both ways, A-to-B and B-to-A) to communicate between and amongst the units. For example, a communication unit employing LPC-10 speech coding can not communicate with a communication unit employing TDVC speech coding unless there is an LPC-to-TDVC transcoder to translate between the two speech coding standards. Many commercial and military communication systems in use today must support multiple coding standards. In many cases, the vocoders are incompatible with each other.
Two conventional solutions that have been implemented to interconnect communication units employing different speech coding algorithms consist of the following:
1) Make all new terminals support all existing algorithms. This “lowest common denominator” approach means that newer terminals cannot take advantage of improved voice quality offered by the advanced features of the newer speech coding algorithms such as TDVC and MELP when communicating with older equipment which uses an older speech coding algorithm such as LPC.                2) Completely decode the incoming bits to analog or digital speech samples from the first speech coding standard, and then reencode the analog speech samples using the second speech coding standard. This process is known a tandem connection. The problem with a tandem connection is that it requires significant computing resources and usually results in a significant loss of both subjective and objective speech quality. A tandem connection is illustrated in FIG. 1. Vocoder decoder 102 and D/A 104 decodes an incoming bit stream representing parametric data of a first speech coding algorithm into an analog speech sample. A/D 106 and vocoder encoder 108 reencodes the analog speech sample into parametric data encoded by a second speech coding algorithm.        
What is needed is a system and method for transcoding compressed speech from a first coding standard to a second coding standard which 1) retains a high degree of speech quality in the transcoding process, 2) takes advantage of the improved voice quality features provided by newer coding standards, and 3) minimizes the use of computing resources. The minimization of computing resources is especially important for space-based transcoders (such as for use in satellite applications) in order to keep power consumption as low as possible.